How to configure the 4th incoming line only for forwarding services, without using it as an additional line?

Question

Computer networks

The problem is the following, we have a central office that has 3 lines of incoming Internet configured.
1) ADSL (on CISCO) - through which there is an exchange on VLAN with other regions.
2) YOTA for use by employees (browser and personal mail).
3) ADSL (zyxel) which is connected to TMG 2010 is used as a gateway for FTP and Terminal 1c
From the management, information was received from above that the new server needs an exchange to a different ip address. (ftp and terminal 1c). On ADSL (zyxel) we have several free ip addresses, I tried to install a simple router and configure everything on it, but problems appeared: as soon as I connect 2 network cable (it doesn’t matter if it’s lan or wan). I lose access to it, both from the external line and from the local network.
Raised the second TMG and did not find any problems. Everything clearly works and does not interfere, but TMG is no longer produced + a sequel and msserver are not much expensive))). Still, a piece of iron for 2-5 thousand is a more economical option.
PS You need not an additional line, but just an external IP address on which these services will work. I apologize for the errors and inaccuracies in the description. Thank you very much for your help.